Lithium polymer secondary battery

ABSTRACT

A lithium polymer secondary battery which comprises a negative electrode, a positive electrode, and polymer electrolyte layers united respectively with the two electrodes and differing in viscoelastic behavior. In this battery, conformation to the expansion and shrinkage accompanying charge/discharge is easy and the interfacial resistance between each electrode and the polymer electrolyte is kept low.

FIELD OF THE INVENTION

[0001] This invention relates to a lithium secondary battery using anion-conductive polymer. More specifically, it relates to a lithiumpolymer secondary battery comprising an anode having an electroactivesubstance comprised of a carbonaceous material capable ofelectrochemically insertion and release of lithium, a cathode having anelectroactive substance comprised of a chalcogenide compound containinglithium, and a polymer electrolyte layer comprised of a matrix of anion-conductive polymer retaining a nonaqueous electrolyte solutiontherein.

BACKGROUND ART

[0002] Lithium secondary batteries have a higher energy density intheory compared to other batteries and thus allow to manufacture a smalland light-weight battery. Therefore, vigorous studies have been focussedthereon to develop a power source of portable electronic instruments.Particularly, performance of such instruments is even increasing inrecent years and their power source is required concominantly therewithto exhibit better discharging characteristics even at a high load. Inorder to fulfill these requirements, various studies are in progressnext to the prior art battery using nonaqueous electrolyte solutionsreferred to as lithium ion battery to develop a battery using a polymerelectrolyte that functions both as the nonaqueous electrolyte solutionand the polymer separator of the prior art battery. Much interest hasbeen focussed to a lithium secondary battery using the polymerelectrolyte because of its remarkable advantages such as the possibilityof making the battery smaller and thinner in size and lighter in weightas well as leak free.

[0003] Generally, secondary batteries now available in the market suchas lithium secondary batteries make use of a nonaqueous electrolytesolution prepared by dissolving an electrolyte salt in an organicsolvent. The use of this solution is problematic because the solution iseasily susceptible to leakage from the battery parts, dissolution ofelectrode substances or vaporization which may develop problems of longterm reliability, spilling off in the sealing process and the like.

[0004] In order to improve these problems, lithium secondary batterieshave been developed which make use of a polymer electrolytemacroscopically occurring as a solid. The polymer electrolyte consistsof a porous matrix of an ion-conductive polymer impregnated with orretaining a nonaqueous electrolyte solution (a lithium salt solution inan aprotic polar organic solvent).

[0005] Microscopically the polymer electrolyte has a continuous phase ofnonaqueous electrolyte solution therein and exhibits a high ionconductivity. This results in a low mechanical strength. The mechanicalstrength may be reinforced by including a separator (porous substrate)in the polymer electrode but another problem still remains to exsist.

[0006] The lithium secondary battery relies on intercalation or dopingof lithium into an electroactive substance which results in expansionand shrinkage of the electroactive layer. If the polymer electrolytefails to accommodate well the expansion/shrinkage, then physical contactbetween the electrode and the polymer electrolyte will becomeunsatisfactory to develop increased interfacial resistance therebetween.This adversely affect the battery perfomance including discharge andcharge cycle characteristics of the battery.

[0007] JP-A-5012913 discloses that the ion conductivity and theelasticity of the polymer electrolyte of this type may well be balancedby increasing the ratio of nonaqueous electrolyte solution toion-conductive polymer to 200% or higher and also increasing theelasticity and elongation of the polymer electrolyte greater thancertain levels. Since greater elasticity levels mean greater strain perunit amount of stress, increased elasticity cannot accommodate both ofexpansion and shrinkage. In order to accommodate both expansion andshrinkage, it is necessary for the polymer electrolyte to have acushon-like property.

[0008] Accordingly, the problem to be solved by the present invention isto provide a lithium secondary battery including a polymer electrolytelayer that can tolerate expansion and shrinkage of the electroactivesubstance layers and exhibit a buffering effect as a whole.

DISCLOSURE OF THE INVENTION

[0009] The present invention provides a lithium polymer secondarybattery comprising an anode having an electroactive substance comprisedof a carbonaceous material capable of electrochemically inclusion andrelease of lithium, a cathode having an electroactive substancecomprised of a chalcogenide compound containing lithium, and a polymerelectrolyte layer sandwiched between the cathode and the anode, whereinsaid polymer electrolyte layer is divided into a sub-layer integrallyformed with said cathode and a sub-layer integrally formed with saidanode, and wherein the sub-layers exhibit different viscoelasticbehavior from each other.

[0010] In a preferred embodiment, the polymer electrolyte sub-layer onthe cathode has an elasticity greater than that of the polymerelectrolyte sub-layer on the anode. This is because theexpansion/shrinkage of the electroactive substance is more remarkable inthe cathode than in the anode and, therefore, a greater mechanicalstrength is required for the polymer electrolyte sub-layer on thecathode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a graph showing the discharge capacity at differentcurrent levels of the battery according to Example 1 of the presentinvention in comparison with the battery according to ComparativeExample 1.

[0012]FIG. 2 is a graph showing the discharge curve at a constantcurrent of 10 mA of the battery according to Example 1 of the presnetinvention in comparison with the battery according to ComparativeExample 1.

[0013]FIG. 3 is a graph showing the discharge curve of the batteryaccording to Example 1 of the present invention in comparison with thebattery according to Comparative Example 1.

BEST MODE FOR CARRYING OUT OF THE INVENTION

[0014] The battery of the present invention may be manufactured byforming an ion-conductive polymer layer separately on a pre-fabricatedcathode and anode and joining the layers together although themanufacturing process is not limited thereto.

[0015] Basically, the anode and cathode comprise a current collector inthe form of a metal foil and an electroactive substance of therespective electrodes bound with a binder material. The materials of thecollector foil include aluminum, stainless steel, titanium, copper,nickel and the like. Aluminum and copper are employed for the cathodeand the anode, respectively in consideration of their electrochemicalstability, ductility and economy.

[0016] Although metal foils are mainly shown herein as the form of anodeand cathode collectors, other forms such as mesh, expanded metals,laths, perforated sheets or plastic films having a coating of anelectron-conductive material may be employed although the form ofcollector is not limited thereto.

[0017] The electroactive substance of the anode is a carbonaceousmaterical capable electrochemically inclusion and release of lithium.Typical examples thereof include particles (flakes, aggregates, fibers,whiskers, beads or ground particles) of natural or artificial graphite.Artificial graphite produced by graphitizing mesocarbon beads, mesophasepitch powder or isotropic pitch powder may also be used.

[0018] With regard to the electroactive substance used in the presentinvention, it is more preferable to use as the carbonaceous materialgraphite particles having attached to the surfaces thereof amorphouscarbon particles. These particles may be obtained by dipping thegraphite particles in a coal-based heavy oil such as pitch or apetroleum-based heavy oil and heating recovered graphite particles to atemperature above the carbonizing temperature to decompose the heavyoil, if necessary, followed by milling. Such treatment significantlyretards the decomposing reaction of the nonaqueous electrolyte solutionand the lithium salt occurring at the anode during the charge cycle toenable the charge and discharge cycle life to be improved and also thegas evolution due to the above decomposition reaction to be prevented.In the above carbonaceous material, micropores contributing to increasein BET specific surface area have been filled with the attached carbonparticles derived from the heavy oil. The specific surface area thereofis generally below 5 m²/g, preferably in the range between 1 to 5 m²/g.Greater specific surface areas are not preferable because increasedcontacting surface area with the ion-conductive polymer makes undesiredside reactions to be taken place more easily.

[0019] The cathodic electroactive substance to be used in the presentinvention in conjunction with the carbonaceous anodic active substanceis preferably selected from a composite oxide of laminar or spinelstructure represented by the formula: Li_(a)(A)_(b)(B)CO₂

[0020] wherein

[0021] A is a transition metal element;

[0022] B is an element selected from the group consisting of a non-metalor semi-metal element of group 3B, 4B and 5B of the periodic chart, analkaline earth metal, Zn, Cu and Ti;

[0023] a, b and c are numbers satisfying the following relationship:

[0024] 0<a≦1.15

[0025] 0.85≦b+c≦1.30, and

[0026] c>0

[0027] Typical examples of the composite oxides include LiCoO₂, LiNiO₂and LiCoxNi_(1-x)O₂ (0<x<1). Use of these compounds in conjunction witha carbonaceous material as an anodic electroactive substance isadvantageous in that the battery exhibits a practically acceptabledynamic voltage even when the voltage variation generated by chargingand discharging the carbonaceous material per se (about 1 volt vs.Li/Li⁺), and that lithium ions necessary for charging and dischargingthe battery are already contained in the form of, for example, LiCoO₂ orLiNiO₂ before assembling the battery.

[0028] When preparing the anode and cathode, the respectiveelectroactive substances may be combined, where necessary, with achemically stable conductor material such as graphite, carbon black,acetylene black, carbon fiber or conductive metal oxides to improve theelectron conductivity thereof.

[0029] The binder is selected among those thermoplastic resins which arechemically stable, soluble in a suitable solvent but hardly attackedwith the nonaqueous electrolyte solution. A variety of suchthermoplastic resins have been known. For example, polyvinylidenefluoride (PVDF) may preferably used since this resin is selectivelysoluble in N-methyl-2-pyrrolidone. Other examples of usablethermoplastic resins include polymers and copolymers of acrylonitrile,methacrylonitrile, vinyl fluoride, chloroprene, vinyl pyridine and itsderivatives, vinylidene chloride, ethylene, propylene and cyclic dienes(e.g. cyclopentadiene, 1,3-cyclohexadiene). A dispersion of the binderresin may also be used in place of a solution.

[0030] The electrode may be produced by kneading the respectiveelectroactive substances and, where necessary, the conductor materialwith a solution of the binder resin to prepare a paste, applying thepaste on a metal foil using a suitable coater to form a film of uniformthickness, and compressing the film after drying. The proportion of thebinder resin in the electroactive substance layer should be minimum andgenerally lies from 1 to 15% by weight. The proportion of the conductormaterial usually lies, when used, from 2 to 15% by weight of theelectroactive substance layer.

[0031] The polymer electrolyte layer is formed on the respectiveelectroactive substance layers thus prepared integrally therewith. Thepolymer electrolyte layer is comprised of a matrix of an ion-conductivepolymer impregnated with or retaining a nonaqueous electrolyte solutioncontaining a lithium salt. The polymer electrolyte layer occursmacroscopically in a solid state but microscopically retains acontinuous phase of the lithium solution formed therein in situ. Thepolymer electrolyte layer of this type has an ion-conductivity higherthan that of the corresponding polymer electrolyte free from the lithiumsolution.

[0032] The polymer electrolyte layer may be formed by polymerizing (heatpolymerization, photopolymerization etc.,) a precursor monomer of theion-conductive polymer in the form of a mixture with the nonaqueouselectrolyte solution containing a lithium salt.

[0033] The monomer component of the above mixture which can be used forthis purpose should include a polyether segment and also bepolyfunctional in respect to the polymerization site so that theresulting polymer forms a three dimensional crosslinked gel structure.Typically, such monomers may be prepared by esterifying the terminalhydroxyl groups with acrylic or methacrylic acid (collectived called“(meth)acrylic acid”). As is well known in the art, polyether polyolsare produced by addition-polymerizing ethylene oxide (EO) alone or incombination with propylene oxide (PO) using an initiator polyhydricalcohol such as ethylene glycol, glycerine or trimethylolpropane. Amonofunctional polyether polyol (meth)acrylate may be used incombination with polyfunctional monomers.

[0034] The poly- and monofunctional monomers are typically representedby the following general formulas:

[0035] wherein R₁ is hydrogen or methyl;

[0036] A₁, A₂ and A₃ are each a polyoxyalkylene chain containing atleast 3 ethylene oxide (EO) units and optionally some propylene oxide(PO) units such that PO/EO=0-5 and EO+PO≧35.

[0037] wherein R₂ and R₃ are hydrogen or methyl;

[0038] A₄ is a polyoxyalkylene chain containing at least 3 EO units andoptionally some PO units such that PO/EO=0-5 and EO+PO≧10.

[0039] wherein R₄ is a lower alkyl, R₅ is hydrogen or methyl, and A₅ isa polyoxyalkylene chain containing at least 3 EO units and optionallysome PO units such that PO/EO=0-5 and EO+PO≧3.

[0040] The nonaqueous electrolyte solution is prepared by dissolving alithium salt in a nonpolar, aprotic organic solvent. Non-limitativeexamples of the lithium salt solutes include LiClO₄LiBF₄, LiAsF₆, LiPF₆,LiI, LiBr, LiCF₃SO₃, LiCF₃CO₂, LiNC(SO₂CF₈)₂, LiN(COCF₃)₂, LiC(SO₂CF₃)₂,LiSCN and mixtures thereof.

[0041] Non-limitative examples of the organic solvents include cycliccarbonate esters such as ethylene carbonate (EC) or propylene carbonate(PC); straight chain carbonate esters such as dimethyl carbonate (DMC),diethyl carbonate (DEC) or ethyl methyl carbonate (EMC); lactones suchas γ-butyrolactone (GBL); esters such as methyl propionate or ethylpropionate; ethers such as tetrahydrofuran and its derivatives,1,3-dioxane, 1,2-dimethoxyethane, or methyl diglyme; nitrites such asacetonitrile or benzonitrile; dioxolane and derivatives thereof;sulfolane and derivatives thereof; and mixtures of these solvents.

[0042] Since the polymer electrolyte on the electrode, particularly onthe carbonaceous material of the anode is required to contain anonaqueous electrolyte solution of which side reactions with thegraphite-based carbonaceous material are retarded, it is preferable touse a solvent system consisting primarily of EC and another solventselected from PC, GBL, EMC, DEC or DMC. For example, a nonaqueouselectrolyte solution containing 3 to 35% by weight of a lithium saltdissolved in the above solvent mixture containing 2 to 50% by weight ofEC exhibits a satisfactory ion conductivity even at low temperatures.

[0043] The proportion of the nonaqueous solution in the mixture with theprecursor monomer should be large enough to maintain the solution ascontinuous phase in the crosslinked polymer electrolyte layer but shouldnot be so excessive to undergo phase separation and bleeding of thesolution from the gel. This can be accomplished by the ratio of themonomer to the electrolyte solution generally within a range from 30/70to 2/98, preferably within a range from 20/80 to 2/98 by weight.

[0044] The polymer electrolyte layer may optionally include a poroussubstrate as a support member. Such substrate may be either amicroporous membrane made from a polymer which is chemically stable inthe nonaqueous electrolyte solution e.g. polypropylene, polyethylene orpolyester, or a sheet (i.e. paper or nonwoven fabric) made from fiber ofsuch poymers. It is preferable, that the substrate has a airpermeability from 1 to 500 sec./cm³ and can retain the polymerelectrolyte therein at a substrate: polymer electrolyte ratio from 91/9to 50:50. This is necessary to achieve an optimum balance between themechanical strength and the ion conductivity.

[0045] When the substrate is not used, the polymer electrolyte layerintegral with the respective electrodes may be fabricated by casting themixture of the precursor monomer and the nonaqueous electrolyte solutionon the respective electroactive substance layers to form a film andpolymerization the monomer in situ. Then both electrodes are joinedtogether with their polymer electrolyte layers facing inwardly.

[0046] When used, the substrate is applied on the electroactivesubstance layer of either one of the electrodes. Then the mixture of theprecursor monomer and the electrolyte solution is cast on the substratefollowed by polymerization of the monomer in situ to form the polymerelectrolyte layer integral with the substrate and the electrode. Thiselectrode is joined together with the other electrode including thepolymer electrolyte layer free of the substrate formed as above withtheir polymer electrolyte layers facing inwardly.

[0047] The above methods are preferred since they insure to form thepolymer electrolyte layer integral with the electrode and the substrate,when used, in a simple manner.

[0048] The mixture of the precursor of ion-conductive polymer (monomer)and the nonaqueous electrolyte solution containing a lithium saltcontains a suitable polymerization initiator depending on thepolymerization method, e.g. a peroxide type or azo type initiator forheat polymerization and a photoinitiator such as acetophenone,benzophenone or phosphine series for photopolymerization. Thepolymerization initiator may be used in an amount from 100 to 1,000 ppmand should not be used in excess.

[0049] According to the presetn invention, the polymer electrolytelayers (sub-layers), formed on the cathode and anode, respectively havedifferent viscoelastic behavior from each other. Generally the polymerelectrolyte is a viscoelastic mass having both properties similar to agenuine elastic solid in which the amount of strain is proportional tothe amount of stress applied and properties similar to a Newton'sviscous liquid in which the speed of deformation is proportional to theamount of stress applied. Therefore, when the polymer electrolyte istested for the stress-strain (elongation) relationship at a constanttensile speed, it behaves like a elastic solid until it reaches ayielding point and thereafter it behaves like a viscous liquid so thatit deforms largely in response to a small increase in the amount ofstress before break. In this test, the larger the amount of exertedenergy before break, the larger the toughness of the test material. Theabove amount of energy may be estimated from the modulus of elasticityof the test material.

[0050] In case of the polymer electrolyte, the modulus of elasticity isa function of the crosslinking density thereof. Therefore, gelledpolymer electrolytes having different modulus of elasticity levels maybe produced, for example, by varying monomer compositions of theion-conductive polymer at a constant ratio of matrix polymer/nonaqueouselectrolyte solution. Accordingly, a polymer gel having a relativelyhigh modulus of elasticity may be obtained, for example, by selecting amonomer composition including a relatively large proportion of a monomerhaving a large number of functionality with regard to polymerizablegroups. This method is merely one of preferred exemplifying methods andother methods for producing polymer electrolytes having differentviscoelastic properties would be obvious to one skilled in the art.

[0051] In a preferred embodiment, the polymer electrolyte sub-layer onthe cathode has a modulus of elasticity in a range from 10⁴ to 10⁶dyne/cm² and the polymer electrolyte sub-layer on the anode has amodulus of elasticity in a range from 10³ to 10⁶ dyne/cm² and themodulus of elasticity of the sub-layer on the cathode is at least 10%greater than that of the sub-layer on the anode. The polymer electrolytelayer as a whole can accommodate well the expansion and shrinkage ofelectroactive substances under the above conditions.

EXAMPLE

[0052] The following Examples are for illustrative purpose only and notintended to limit the scope of the present invention thereto.

Example 1

[0053] 1) Fabrication of Anode

[0054] As a carbonaceous material, a particulate graphite havingamorphous carbon microparticles attached to the surfaces of graphiteparticles was used. The graphite particles have a d002 value of 0.336 nmdetermined according to the large angle X-ray diffraction method, an Lcvalue of 100 nm, an La value of 97 nm and a BET specific surface area of2 m²/g.

[0055] A blend of 100 weight parts of the above carbonaceous materialand 9 weight parts of polyvinylidene fluoride (PVDF) was kneaded with anamount of N-methyl-pyrrolidone (NMP). The resulting paste was appliedonto a rolled copper foil of 20 μm thickness, dried and compressed. Thesurface area of the cathode was 9 cm² and the thickness thereof was 85μm.

[0056] 2) Nonaqueous Electrolyte Solution/Monomer Mixture

[0057] LiBF₄ was dissolved at a concentration of 1.0 mol/L in a mixtureof ethylene carbonate (EC):propylene carbonate (PC): γ-butyrolactone(GBL): ethyl methyl carbonate (EMC)=30:20:30:20 by volume.

[0058] To 95 weight parts of this solution were added 2.5 weight partsof a trifunctional polyether polyol triacrylate (MW=7500-9000) of theformula:

[0059] wherein A₁, A₂ and A3 are each polyoxyalkylene chain containingat least 3 EO units and at least one PO unit in PO/EO ratio of 0.25; and2.5 weight parts of triethylene glycol methyl ether acrylate of theformula:

[0060] Then 1,000 ppm of 2,2-dimethoxy-2-phenylacetophenone (DMPA) wasadded to prepare a polymerization liquid.

[0061] 3) Fabrication of Polymer Electrolyte Layer Integral with Anodeand Separator Substrate

[0062] The above polymerization liquid was cast on the electroactivesubstance layer of the anode.

[0063] A polyester nonwoven fabric having an air permeability of 380sec/cm³, a thickness of 20 μm and an area of 10 cm² was placed on theanode and the above polymerization liquid was poured thereon in anamount sufficient to reach a fabric: liquid ratio=90:10 by weight. Thenthe anode-fabric stack was irradiated with UV radiation of 365 nmwavelength at an intensity of 30 mW/cm² for 3 minutes to form a gelledpolymer electrolyte layer integrally with the anode and the nonwovenfabric. The thickness of the polymer electrolyte layer was 20 μm.

[0064] 4) Fabrication of Anode

[0065] 100 weight parts of LiCoO₂ having an average particle size of 7μm and 5 weight parts of acetylene black, and 5 weight parts of PVDFwere blended and kneaded with an amount of NMP. The resulting paste wasapplied on a rolled aluminum foil of 20 μm thickness, dried andcompressed. The area and the thickness of cathode were 80 μm and 9 cm²,respectively.

[0066] 5) Polymerization Liquid on Cathode

[0067] LiBF₄ was dissolved at a concentration of 1.0 mol/L in a mixtureof ethylene carbonate (EC): propylene carbonate (PC): γ-butyrolactone(GBL): ethyl methyl carbonate (EMC)=30:20:30:20 by volume.

[0068] To 90 weight parts of this solution were added 5 weight parts ofa trifunctional polyether polyol triacrylate as used on the anode sideand 1,000 ppm of DMPA to obtain a polymerization liquid.

[0069] 6) Fabrication of Polymer Electrolyte Layer on Cathode

[0070] The above polymerization liquid was cast on the cathode andirradiated with UV radiation of 365 nm wavelength at an intensity of 30mW/cm² for three minutes to fabricate a polymer electrolyte layerintegrally with the cathode. The thickness of the polymer electrolytelayer was 10 μm.

[0071] 6) Assembly of Battery.

[0072] The cathode and the anode as prepared above were joined togetherwith their polymer electrolyte layers facing inwardly to assemble abattery having a total thickness of 190 μm.

Comparative Example 1

[0073] Example 1 was repeated except that the polymer electrolytesub-layer of Example 1 was used in both of the cathode and the anode.

[0074] The polymer electrolyte sub-layer of the anode in Example 1 (sameas the polymer electrolyte sub-layers on the cathode and the anode inComparative Example 1) was tested for the modulus of elasticityaccording to the method described below. The same test was alsoconducted for the cathode polymer electrolyte sub-layer on the cathodeof Example 1.

[0075] The respective precursor liquids were cast on a stainless steelfoil and irradiated with UV radiation of 365 nm wavelength at anintensity of 30 mW/cm² for 3 minutes to form a gelled polymerelectrolyte sheet having a thickness of 500 μm. A sample was taken fromthis sheet and the modulus of elasticity thereof was determined using adynamic viscoelastometer. The results are shown in Table 1 below.

[0076] Evaluation of Battery Performance:

[0077] The batteries of Example 1 and Comparative Example 1 were chargedat a constant current of 4.0 mA until the battery voltage reached 4.1 Vand the charged at a constant voltage for a total pre-charge time of 12hours. The charged batteries were then discharged at different constantcurrent levels of 2, 3, 5, 10 and 20 mA until the battery voltagedecreased to 2.75 V. The discharge capacities at different dischargecurrent levels are shown in the graph of FIG. 1.

[0078] Similarly, the batteries were charged under same conditions asabove and were discharged at a constant current level of 10 mA until thebattery voltage decreased to 2.75 V. The discharge curves of therespective batteries are shown in the graph of FIG. 2.

[0079] Similarly, the batteries were charged under same condition asabove and then discharged at a constant current level of 2.3 mA untilthe battery voltage reached 2.75 V. This charge-discharge cycle wasrepeated a number of times for testing the variation in dischargecapacity. The results are shown in the graph of FIG. 3.

[0080] As the results of these tests show, the battery of Example 1 issuperior to the battery of Comparative Example 1 in terms of thedischarge capacity at different current levels, the discharge capacitycharacteristics under a high load and the charge-discharge cyclingcharacteristics. It is postulated that these results are attributed tothe fact that the polymer electrolyte layer of the battery of Example 1as a whole can easily accommodate the expansion/shrinkage of the cathodeand anode during the charge and discharge cycles compared to the polymerelectrolyte layer of the battery of Comparative Example 1 to keep theinterfacial resistance between the polymer electrolyte layer and theelectrode at a minimum level.

[0081] Separately, the batteries of Example 1 and Comparative Example 1were tested, respectively for the number of incident of microshortcircuit in 20 batteries immediately after manufacutre. The results arealso shown in Table 1. It is also postulated that the fewer number ofincident of microshort circuit in the battery of Example 1 compared tothe battery of Comparative Example 1 is attributed to the improvedmechanical property of the polymer electrolyte layers a whole. TABLE 1Example 1 Comp. Ex. 1 Item Cathode Anode Cathode Anode Modulus of 2.18 ×10⁵ 8.9 × 10³ 8.9 × 10³ 8.9 × 10³ elasticity, 25° C. (dyne/cm²) Incidentof 0/20 2/20 microshort circuit

1. (Amended) A lithium polymer secondary battery comprising an anode having an electroactive substance comprised of a carbonaceous substance capable of electrochemically inclusion and release of lithium, a cathode having an electroactive substance comprised of a chalcogenide compound containing lithium, and a polymer electrolyte layer sandwiched between the cathode and the anode, wherein said polymer electrolyte layer is divided into a sub-layer integrally formed with said anode and a sub-layer integrally formed with said cathode, and wherein said sub-layers exhibit different viscoelastic behavior from each other by having different crosslinking densities from each other.
 2. The lithium polymer secondary battery according to claim 1 wherein said polymer electrolyte sub-layer on said cathode has a modulus of elasticity which is at least 10% greater than that of the polymer electrolyte sub-layer on said anode when the parameter of said viscoelastic behavior is expressed as the modulus of elasticity.
 3. (Cancelled)
 4. (Amended) The lithium polymer secondary battery according to claim 1 wherein said ion-conductive polymer is a homo- or copolymer of a polyether polyol poly(meth)acrylate containing an ethylene oxide unit and optionally a propylene oxide (PO) unit in the polyether chain thereof, and wherein said ion-conductive polymer in said sub-layer formed on the cathode has a precursor monomer composition thereof that gives a crosslinking density which is higher than the crosslinking density of the corresponding precursor monomer composition in said sub-layer on the anode.
 5. The lithium polymer secondary battery according to claim 4 wherein the solvent of said nonaqueous electrolyte solution is selected from ethylene carbonate, propylene carbonate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone or a mixtutre thereof.
 6. The lithium polymer secondary battery according to claim 1 wherein said electroactive substance of said anode is a particulate graphite having amorphous carbon attached to the surfaces thereof. 